= Solution
Above-gap <photon absorption> generates <electrons> and <electron holes>. A <p-n junction>'s built-in field and carrier diffusion direct them toward selective contacts; their nonequilibrium populations can maintain separated quasi-Fermi levels and deliver electrical <power>. The equilibrium built-in <voltage> alone is not a battery supplying continuous <power>.
Choose diode current positive into the cell. An idealized characteristic is
$$
I_{\rm dark}=I_0(e^{eV/(nk_BT)}-1),\qquad
I_{\rm light}=I_{\rm dark}-I_{\rm ph}.
$$
Illumination shifts the curve downward. At short circuit $I(0)=-I_{\rm sc}$; at open circuit $I(V_{\rm oc})=0$. Between those points, $V>0$ and $I<0$, so the delivered <power> $P=-VI$ is positive.
\Image[/past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2018/paper-1-photovoltaics.png]
{title=Original idealized photovoltaic current–voltage curves and the descending-bandgap order of the proposed three-junction stack}
\b[The <solar-cell fill factor> compares maximum delivered <power> with the open/short-circuit rectangle]:
$$
\boxed{\mathrm{FF}=\frac{V_{\rm mp}|I_{\rm mp}|}{V_{\rm oc}I_{\rm sc}}.}
$$
Series resistance, leakage, <recombination> and <temperature> change the shape and hence the fill factor. Neither the short-circuit nor the open-circuit point itself delivers <power>.
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